Portable devices such as mobile telephones and tablet computers are usually battery operated and equipped with a charging connector for receiving charging power from a plug of an external charger. Recently, wireless charging has been made available with most sophisticated mobile telephones. With wireless charging, a mobile telephone can be laid onto a charging pillow that contains a coil and necessary circuitry for causing the coil to emit a variable magnetic field. The mobile telephone can obtain direct current from the variable magnetic field using its own coil and rectifier circuitry.